1. Field of the Invention
The invention pertains to a device for the measurement, in a wide range, of a non-reciprocal phase shift generated in a ring interferometer, and its measuring method.
More particularly, it pertains to an optic fiber gyrometer and a method for the measurement of rotational speed.
2. Description of the Prior Art
An essential improvement in optic fiber gyrometers has been described in the French Pat. No. 84 09311. This patent describes an optic fiber gyrometer with a phase shifting device. The phase shift control is obtained by a feedback loop or, preferably, by a dual feedback loop, the second loop performing corrections of the quantification by the first loop. The device described in the French Pat. No. FR-84 09311 can be used to work on the maximum energy of an interference fringe and, thus, to have improved sensitivity and signal-to-noise ratio. However, the phase shift introduced cannot increase indefinitely. Thus, the phase shift has a periodic variation.
The device described in the French Pat. No. FR-84 09311 works perfectly for a limited range of angular speed. The feedback loops enable compensation for low rotation speeds. Excessive rotation speeds could cause the automatic control or feedback device to be locked no longer into the 0 order interference fringe but into a following interference fringe. The device continues to "work" perfectly except that the phase shift between the interference fringe on which the automatic control really takes place and the central (0 order) interference fringe is not taken into account. It follows that, outside the range of operation of the gyrometer, the measurements are false. Furthermore, this locking-in could become stable, causing the measurements to be false for a long period.
The device according to the present invention can be used to obtain the measurement precision of the device described in the patent FR-84 09311 while, at the same time, very greatly extending the range of measurable values. To do so, a device according to the present invention has means by which that interference fringe on which the operation is being done, namely the current interference fringe, can be determined. In this case, locking into a different fringe no longer has the drawbacks inasmuch as, in the determining of the speed, the additional phase shift, introduced by the fact that the central fringe is not worked on, is taken into account. The measurements of the order of the interference fringe, to which the feedback loops are locked, are done, for example, by continuously or periodically comparing the measurements for the different frequencies. For different frequencies, only the zero of the central fringe is stable. The variation in the modulation of the zero with the frequency makes it possible to ascertain the current interference fringe and, consequently, to achieve the compensation needed to determine the precise rotation speed.